During the 35 to 50 years' service life of structures such as steel pipelines and fittings for transportation of oil and gas products, they are exposed to variations in operational conditions ranging from those of ordinary onshore applications to those of more severe offshore underwater applications. To protect the structures against corrosion from exposure to such operational conditions, the structures are typically coated with multilayer coatings. The multilayer coatings must be durable in that they must maintain adhesion, resist moisture- and gas ingress, maintain mechanical performance, and must meet increasingly stringent environmental and safety regulations.
Typically, the multilayer coatings are polyolefin-based, and comprise a primer layer, an intermediary adhesive layer and a topcoat layer. The primer layer, typically epoxy-based, provides protection to the inner metallic structure against chemical and oxidative corrosion, and enhances adhesion between the coating and the metallic structure. The adhesive layer, typically polyolefin-based, promotes compatibility between the primer layer and the topcoat layer so that strong interfacial mechanical bonds are formed at the interface when brought together in the molten state. The topcoat layer provides protection against mechanical damage arising from the handling, transportation and laying of the structure in service.
Over time, the primer layer absorbs moisture, which predisposes it to damage during handling. In particular, epoxy-based primer layers, due to their highly crosslinked structure, are inherently brittle and susceptible to microcrack formation deep within its structure where detection and intervention are often difficult or impossible. Therefore, throughout the service life of multilayer coatings, fractures within the primer layer may occur, which compromises the structural integrity of the coating.
The development of “mendable” epoxy resins has been proposed in recent years to restore adhesive strength to a fractured component. Such “mendable” epoxy resins may be dispersed with brittle vessels containing liquid healing agent(s) which rupture upon propagation of the fracture to release the healing agent(s), which in turn fills up and rejoins the fractured surfaces. Applications of such “mendable” epoxy resins in multilayer coatings have been limited by high raw material cost, lack of long-term stability of the healing agent(s), additional steps required to encapsulate the healing agent(s) into delivery vessels, and lack of ability to heal multiple incidents of damage.
In multilayer coatings, adhesive or cohesive failure between the primer and the topcoat layers can also create a cavity between, or that traverses the layers which have differing properties. The primer layer (typically epoxy-based) has an amorphous cross-linked network and is relatively polar, while the topcoat layer (typically polyolefin-based) has a semi-crystalline structure and is relatively non-polar. Due to the differing properties of the different layers in a multilayer coating, simply heating the surrounding topcoat to melt the materials does not ensure that the interlayer cavity is refilled and the fractured surfaces rejoined and healed. When heated beyond the melting point of the polymers, the lack of any hierarchical structure may result in the amorphous polymer simply melting and flowing away without any direction instead of refilling the cavity to rejoin the fractured surfaces and healing the multilayer coating.
There is a need to provide a polymer composition that overcomes, or at least ameliorates, one or more of the disadvantages described above.
There is a need to provide a polymer composition for use in multilayer coatings that is able to self-heal fractures that form within a layer, and/or fractures that form between and/or that traverses the different layers in the coating that may have differing properties in order to restore structural integrity to the coating, without compromising its impact strength, adhesive performance, temperature performance and durability.
There is further a need to provide a method for preparing a polymer composition that is able to self-heal for use in multilayer coatings, that is cost-effective and meet environmental and health regulations, and that maximizes modification of the composition without causing degradation of the polymer.